Variable sensitivity interferometer systems

ABSTRACT

Variable Sensitivity optical sensors can have a respective actual sensitivity of one or more portions of the sensor corresponding, at least in part, to a selected environment of each respective sensor portion. Some disclosed sensors have a plurality of optical conduits extending longitudinally of the sensors. At least one of the optical conduits can have at least one longitudinally extending segment having one or more optical and/or mechanical properties that differs from the optical properties of an adjacent longitudinally extending segment, providing the conduit with longitudinally varying signal propagation characteristics. An optical sensor having such optical conduits can exhibit a longitudinally varying actual sensitivity. Nonetheless, such a sensor can exhibit a substantially constant apparent sensitivity, e.g., when each respective portion of the sensor exhibits an actual sensitivity corresponding to a selected environment. Innovative sensors can provide a low-incidence of false or nuisance alarms, accurate position and magnitude information, and other advantages.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application Ser. No. 61/393,298 and U.S. Provisional PatentApplication Ser. No. 61/393,321 (referred to hereinafter as the “SystemPatent Application”), both filed Oct. 14, 2010, the contents of whichare hereby incorporated by reference as if recited in full herein forall purposes.

BACKGROUND

The innovations disclosed herein pertain to interferometer systems, andmore particularly, but not exclusively, to fiber-optic interferometersystems having variable-sensitivity sensors. Some innovativeinterferometer systems are configured to detect and/or locatedisturbances (e.g., a disturbance to a secure perimeter, such as a “tap”on a fence, a leak from a pipeline, a change in structural integrity ofa bridge, a disturbance to a communication line, a change in operationof a conveyor belt, an impact on a surface or acoustical noise, amongothers). In some instances, such systems can be configured to detectand/or locate over distances up to, for example, about 65 kilometers(km) with one passive sensor, and up to, for example, about 130 km withfirst and second passive sensors extending in opposite directions.

Most optical conduits (e.g., optical fibers) have substantially uniform(e.g., unvarying) properties along their lengths, making such opticalconduits suitable for a wide variety of applications (e.g.,communications, interferometers) that demand homogeneous properties.This homogeneity is a result of, among many factors, modern high-qualitymanufacturing processes for optical fibers, coatings on the opticalfibers and various protective sheaths of the cable in which the fibersare typically encased. When used as a sensor configured to detect adisturbance, such longitudinally homogeneous optical conduits typicallyrespond to a given perturbation in a uniform manner, regardless of wherethe perturbation is applied along the sensor's length.

In many applications, different portions of a given sensor can beexposed to respective different environments. For example, a portion ofa sensor can be positioned, for example, under water, another portioncan be positioned underground and yet another portion can be positionedabove-ground (e.g., exposed to the atmosphere). In such an application,known sensors can respond to a given disturbance differently depending,for example, on the environment and which portion of the sensor isperturbed. Therefore, with known sensors having homogeneous sensitivity,it can be difficult to discern one or more characteristics (e.g.,amplitude, position, etc.) of any particular disturbance, particularlyif the environmental surroundings vary along the sensor's length.Accordingly, known fiber-optic sensors can be prone to initiating“false” or “nuisance” alarms. Although some environmental effects can befiltered mathematically to reduce a rate of false and nuisance alarms,such algorithms can be computationally intensive and can lead tointermittent operation. Moreover, such mathematical filtering may notsatisfactorily reduce the occurrence of false or nuisance alarms.

Accordingly, there remains a need for sensors, e.g., passive fiber-opticsensors, configured to extend through more than one environment whileresponding similarly to a given disturbance regardless of theenvironment. Other needs relating to sensing systems are also unmet.

SUMMARY

Innovative optical sensors and related interferometer systems addressingone or more of the above-identified and other needs are disclosed. Someembodiments of such innovations include a sensor having an actualsensitivity that varies along its length.

A sensor having substantially constant properties along its lengthtypically has a substantially constant actual sensitivity along itslength. A given disturbance can be conveyed to a sensor through oneenvironment differently than through another environment, making asensor's response to such a disturbance appear to be environmentallydependent. Moreover, sensors with longitudinally uniform propertiesexhibit an apparent sensitivity in one portion exposed to a givenenvironment that differs from an apparent sensitivity exhibited byanother portion of the sensor positioned in another environment. As usedherein, “actual sensitivity” means a measure of a sensor's response to agiven disturbance in a selected reference environment. As used herein,“apparent sensitivity” means a measure of a sensor's response to a givendisturbance in an arbitrary environment. For example, a singlemodeinterferometer buried in the ground might produce 10 interferencefringes in response to a given physical disturbance. The sameinterferometer (or a portion thereof) positioned above-ground mightproduce 500 interference fringes in response to a similar disturbance.

In contrast to a sensor having longitudinally homogeneous opticalproperties, a sensor having longitudinally varying optical properties,and a corresponding longitudinally varying actual sensitivity, canprovide a substantially constant apparent sensitivity when the sensorextends through a variety of environments. Innovative optical sensorsare disclosed in which the respective actual sensitivity of one or moreportions of the sensor correspond, at least in part, to a selectedenvironment of the respective sensor portions.

For example, some disclosed sensors have a plurality of optical conduitsextending longitudinally of the sensors. At least one of the opticalconduits can have at least one longitudinally extending segment havingone or more optical and/or mechanical properties (e.g., birefringence,fiber coating, sheaths, etc.) that differ from the optical properties ofan adjacent longitudinally extending segment, thus providing the conduitwith longitudinally varying signal propagation characteristics. Anoptical sensor having one or more such optical conduits can exhibit alongitudinally varying actual sensitivity. Nonetheless, such a sensorcan exhibit a substantially constant apparent sensitivity, such as whenthe sensor extends through a plurality of environments (e.g., as apipeline can), particularly when each respective portion of the sensorexhibits an actual sensitivity corresponding to a selected environment.Such an innovative sensor can provide a low-incidence of false ornuisance alarms, as well as accurate position and magnitude information.

Some innovative systems comprise a method for detecting a disturbancewith a sensor having a position-dependent actual sensitivity. With someembodiments of such innovative systems, a position of a disturbance canbe determined, and, in some instances, a magnitude of such a disturbancecan also be determined when the sensor spans a variety of environments(e.g., above ground, below ground, underwater and through openatmosphere).

Some disclosed sensors are passively terminated and are configured toextend, for example, up to and even more than about 50 km away fromactive components. Some disclosed systems have two such sensorsextending from the active components in opposite directions relative toeach other, providing a disturbance-detection capability over largedistances, for example, up to about 100-130 km.

The foregoing and other features and advantages will become moreapparent from the following detailed description, which proceeds withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings show aspects of the innovative systemsdisclosed herein, unless specifically identified as showing a knownfeature from the prior art.

FIG. 1 shows aspects of an innovative interferometer of the typedisclosed herein.

FIG. 2 shows aspects of another innovative interferometer system of thetype disclosed herein.

FIG. 3 shows a schematic illustration of a commercially available MachZehnder interferometer configured to use counter-propagating opticalsignals having actively matched polarization states.

FIG. 4 shows aspects of a sensor configured to provide an actualsensitivity that varies with longitudinal position; a portion of thesensor is shown in an enlarged view.

FIG. 5A shows a cross-sectional view taken along line 5A-5A in FIG. 4.FIG. 5B shows the cross-sectional view in FIG. 5A with several differentpairings of optical conduits identified. Each unique combinationprovides the optical cable with a corresponding unique actualsensitivity.

FIG. 6 shows a cross-sectional view taken along line 6-6 in FIG. 4.

FIG. 7 shows a sensor of the type disclosed herein extending amongvarious environments, as when installed, for example, to monitor apipeline.

DETAILED DESCRIPTION

Various principles related to optical conduits and interferometersystems are described herein by way of reference to exemplaryembodiments. One or more of the disclosed principles can be incorporatedin various configurations to achieve one or more performancecharacteristics. Disclosed embodiments of optical conduits andinterferometer systems relating to perimeter security applications aremerely examples used to illustrate one or more of the innovativeprinciples described herein. Some embodiments may be equally applicableto use in many other applications, such as, for example, detecting aleak in a pipeline, detecting a failure in a structure, detecting adisturbance to a ground surface, detecting a change in operation of aconveyor, etc.

Some innovative optical conduits disclosed herein can be combined withknown interferometer configurations to provide levels of performancethat heretofore have been unachievable. Examples of such innovativecombinations are described below.

Overview of Interferometer Systems

Interferometer systems as disclosed herein can detect a disturbance to asensor portion by comparing a phase shift between observed first andsecond optical signals that have travelled through a first (e.g., a“reference”) optical conduit and a second (e.g., a “sensor”) opticalconduit. In systems disclosed herein, one or more optical and/ormechanical properties differ between the first optical conduit and thesecond optical conduit.

For example, the innovative interferometer 100 shown in FIG. 1 isconfigured as a hybrid Michelson/Mach-Zehnder interferometer having anactive portion 132 a, as disclosed in the System Patent Applicationidentified above, and a passive portion 130 a. The interferometer 100 ashown in FIG. 2 is also configured as a hybrid Michelson/Mach-Zehnderinterferometer having an active portion 132 b that includes apolarization scrambler, and a passive portion 130 b. As shown in FIGS. 1and 2, the systems 100, 100 a include respective passive optical sensorportions 130 a, 130 b extending away from the respective active portions132 a, 132 b toward a distal end 118.

FIG. 3 shows an interferometer system having overlapping first andsecond Mach-Zehnder interferometers configured to convey counterpropagating optical signals.

A disturbance to one or both of the optical conduits 114 a, 114 b (or114 a′, 114 b′, or the sensor portion of the overlapping Mach-Zehnderinterferometers shown in FIG. 3) can modify each respective opticalsignal conveyed through the disturbed conduit. By observing such amodified signal, the existence of such a disturbance can be detected,and, in some instances, the position and magnitude of the disturbancecan be identified.

Overview of Optical Sensors

In some instances, the first and second optical conduits 114 a, 114 b(FIG. 1), 114 a′, 114 b′ (FIG. 2) can have similar optical and/ormechanical properties and similar lengths. In some embodiments, thereference and sensor optical conduits are physically separate conduitspositioned adjacent each other in a “bundle” (also referred to as a“cable”).

For example, a conventional fiber optic bundle can include severalindividual optical fibers (e.g., single-mode fibers) shrouded by anouter sheath(s). One of the individual optical fibers can define thesensor conduit (e.g., 114 a) and another of the individual opticalfibers can define the reference conduit (e.g., 114 b). Yet another ofthe individual optical fibers can define a return conduit, such as in apassively terminated sensor as disclosed in System Patent Applicationidentified above. Respective fibers defining the sensor, reference andreturn conduits can be positioned within and shrouded by the commonouter sheath(s). Although such optical fibers are usually positionedrelatively close to each other (e.g., within several millimeters of eachother), a load or other force that alters the optical phase of thesignals in the individual optical conduits (e.g., an impact orperturbation) applied to the outer sheath will be transmitted to each ofthe individual fibers slightly differently. Moreover, each of theindividual fibers can respond (e.g., deform or momentarily have itsrefractive index changed) to identical loads somewhat differently. Thus,in practice, a disturbance to the cable generally will perturb thereference and the signal conduits 114 a, 114 b differently.

Since physical responses typically differ between the “sensor” conduitand the “reference” conduit, light travelling through the “sensor”conduit can arrive at a terminal end of the sensor conduit (FIGS. 1, 2and 3) at a slightly different time, and possibly with a differentpolarization state, than light travelling through the “reference”conduit. Thus, optical signals observed at each respective terminal endwill usually be out of phase from each other by some amount. When eitheror both of the sensor and reference conduits has been disturbed, therelative phase of the optical signals observed at each respectiveterminal end will tend to shift from the nominal level from theundisturbed conduits. By comparing a delay between the first of theoptical signals and the second of the optical signals (e.g., an observedphase-shift between the signals), and accounting for characteristics ofthe interferometer components (e.g., lengths of optical conduits, speedof light through the conduits, optical wavelength), the magnitude andposition of a disturbance can be determined using a system as shown inFIGS. 1, 2 and 3.

Although many factors can cause an observed phase shift between signalsconveyed through the first and second optical conduits, a nominal, orbaseline, phase shift between observed signals of undisturbed referenceand sensor conduits can be determined. Thus, one can infer that a sensorcable (e.g., a bundle having a sensor conduit and a reference conduit)has been disturbed when a sufficiently large (or a threshold) deviationfrom a baseline phase shift is observed. In addition, observing such aphase-shift at more than one location through the optical path, combinedwith characteristics of the sensor cable (e.g., its length, the speed atwhich light travels through each of the optical conduits, opticalwavelength), a location of the disturbance can be inferred.

As noted above, in some embodiments, a third, insensitive conduit can bepositioned adjacent one or both of the sensor conduit (e.g., conduit 114a) and the reference conduit (e.g., conduit 114 b). For example, anoptical cable can have a plurality of optical conduits within a commonsheath(s), as described above, and shown in FIG. 5A.

Environmental Dependency of Optical Sensor Response

As described above, a disturbance (e.g., an impact or perturbation)applied to the outer sheath of a cable typically will be transmitted toeach of the individual optical conduits slightly differently. Inaddition, a disturbance to an environment surrounding or adjacent to asensor cable 130 a, 130 b, 130 c can be transmitted to the cabledifferently in one environment than in another environment. For example,a load transmitted to an underground sensor and arising from a givendisturbance (e.g., someone digging a hole adjacent the undergroundsensor) typically differs from a load transmitted to an above-groundsensor arising from the same disturbance. Accordingly, a disturbance toeach respective optical conduit in a sensor typically corresponds, atleast in part, to the environment through which the sensor extends.

As a consequence, effects of such a disturbance on an optical signalpropagating through the respective optical conduits also correspond, atleast in part, to the environment. It is believed that such effects atleast partially contribute to observed variations in apparentsensitivity for a given sensor with longitudinally uniform propertiesextending through different environments.

Sensors Having Longitudinally Varying Actual Sensitivity

As noted above, an optical sensor with a substantially constant actualsensitivity along its length can respond to disturbances in differentenvironments differently, making it difficult to discern whether anobserved event corresponds to a disturbance of the type intended to besensed with systems of the types shown in FIGS. 1, 2 and 3. Accordingly,such an optical sensor can be prone to initiating “false” or “nuisance”alarms when the sensor extends through more than one environment.Although some false or nuisance alarms can be filtered mathematically,such algorithms can be computationally intensive and can lead tointermittent operation, without satisfactorily reducing the occurrenceof false or nuisance alarms.

As explained above, differing physical responses between a selected pairof optical conduits (e.g., the conduits 114 a, 114 b) can significantlyaffect an optical signal travelling through the respective conduitsdifferently. An observed difference between such optical signals can beused to detect a disturbance to one or both of the conduits. As usedherein, a “reference-sensor pair” means a selected pair of opticalconduits (e.g., 114 a, 114 b) configured to convey respective opticalsignals and to operatively couple to one or more components (e.g., 132a, 132 b) configured to respond to one or both of the optical signals.

Fiber optic sensors having longitudinally varying sensitivity are nowdescribed. For example, a distance between selected optical conduitsforming a reference-sensor pair of conduits, a construction of each inthe pair of conduits, or both, can vary along the sensor's length. Otherphysical characteristics (e.g., length, birefringence, sheathconstruction, cable fill material, polarization) can also vary along thesensor's length and provide a longitudinally varying physical responsebetween selected reference-sensor pairs. FIG. 4 schematicallyillustrates one example 230 of an optical sensor having an actualsensitivity that varies longitudinally. FIGS. 5A and 5B show asix-bundle cable capable of providing at least five different actualsensitivities, as described more fully below.

In FIG. 4, the sensor 230 extends between a proximal end 231 configuredto operatively couple to an active portion of an interferometer (e.g.,an interferometer 100, 100 a, 100 b shown in FIGS. 1 through 3,respectively) and a distal end 232. As noted above regarding the sensors130 a, 130 b, 130 c, the sensor 230 can be passively terminated at ornear its distal end 232.

The illustrated sensor 230 includes four segments 233, 235, 237, 239,each being configured to provide a respective actual sensitivity todisturbances. In particular, a first segment 233 extends between theproximal end 231 and a first joint 234; a second segment 235 extendsbetween the first joint 234 and a second joint 236; a third segment 237extends between the second joint 236 and a third joint 238; and a fourthsegment 239 extends between the third joint 238 and the distal end 232.As will now be described, each of the first, second, third and fourthsegments of the sensor 230 can be operatively configured to provide acorresponding unique actual sensitivity.

Each of the illustrated segments 233, 235, 237, 239 has a substantiallyidentical construction. As can be seen in FIG. 5A, the segment 235includes six optical conduits 241, 242, 243, 244, 246, 247, eachextending longitudinally of the segment; two such longitudinallyextending conduits 243, 244 are shown in FIG. 6. In each segment shownin FIG. 4, the optical conduits (e.g., optical bundles) arecircumferentially spaced from each other (e.g., at about 60-degrees fromeach other) around a central, longitudinal axis of the cable. Four ofthe six conduits, i.e., conduits 243, 244, 246 and 247, includetight-buffered fibers and are arranged in opposing pairs relative to thecentral longitudinal axis. The other two conduits 241, 242 includeloose-tube fibers and are positioned about 180-degrees from each other,each being positioned between two respective of the conduits 243, 244,246 and 247 having tight-buffered fibers.

Each of the six optical conduits can include at least one single-modeoptical conduit. As used herein, “tight-buffered fibers” means a groupof longitudinally extending optical fibers that are tightly packed (orheld) into an operative structure and surrounding by dry materials. Thetight-buffer fibers typically have a 900 micron outer diameter. As usedherein, “loose-tube fibers” means a group of longitudinally extendingoptical fibers that are free-floating in a viscous (e.g., Newtonian)fluid, a gel, or a non-Newtonian fluid inside a dedicated fiber housingwithin the cable. In some configurations, such a housing can be a tube.Such a gel or fluid (e.g., Newtonian or non-Newtonian) can tend todampen a disturbance to the loose-tube fibers. The loose-tube fiberstypically have a 250 micron outer diameter. Each fiber housing encasingthe loose-tube fibers can have a plurality of fibers within, such as,for example, 6 or 12 fibers. Tight-buffered fibers are typically moreresponsive to a disturbance than loose-tube fibers.

In FIG. 5, an outer sheath 251 extends longitudinally of each segment233, 235, 237, 239. Interstitial spaces 250, 250 a in each segment canbe filled with a suitable strengthening, packing and/or protectivematerial. In some instances, such a suitable fill includesfiber-reinforced plastic, Kevlar fibers, water absorbing fabrics/tapesor other materials. In some instances, the interstitial spaces arefilled with a material that dampens disturbances to the sheath 251, andin other instances the interstitial spaces are filled with a materialthat conveys such disturbances with minimal losses.

Each segment 233, 235, 237, 239 has a respective reference-sensor pairof optical conduits. FIG. 5B shows several possible reference-sensorpairs from which the respective reference-sensor pair can be selected.For example, a reference-sensor pair for a given segment 233, 235, 237,239 can be formed from: (1) opposing tight-buffered fibers 243, 244; (2)opposing loose-tube fibers 241, 242; (3) loose-tube fibers 241 andadjacent tight-buffered fibers 243; (4) adjacent tight-buffered fibers243, 246; or (5) a pair of loose-tube fibers 241 or 242 within a commonloose-tube housing.

Since the actual sensitivity relates, at least in part, to a distanceseparating the respective reference-sensor pair and the construction ofeach in the pair of fibers, each segment 233, 235, 237, 239 can have aunique actual sensitivity even though each segment has a substantiallyidentical overall construction. For example, segment 233 can have thefirst (1) reference-sensor pair, segment 235 can have the second (2)reference-sensor pair, segment 237 can have the third (3)reference-sensor pair, and segment 239 can have the fourth (4)reference-sensor pair. Each of these reference-sensor pairs can beexpected to differ in response to a disturbance, but each segment has asubstantially identical construction, as explained above.

In FIG. 6, a portion 240 of the sensor is shown in longitudinalcross-section. Adjacent segments 235, 237 having substantially identicalconstruction are shown, although the reference-sensor pairs differ ineach segment, as just described. Nonetheless, the segment 235 can beoperatively coupled to the adjacent segment 237 using a conventionaloptical joint (e.g., fusion splice, mechanical splice, butt splice,etc.) 245 a, 245 b configured to join longitudinally adjacent conduits(e.g., to join conduit 241 to conduit 243 and conduit 242 to conduit244, respectively). Such a joint can be an optical fiber fusion spliceor a mechanical coupling.

Operatively joined conduits can have different constructions. Forexample, conduit 241 can be a loose-tube optical fiber and conduit 243can be a tight-buffered fiber. With such a construction, the portion 240of the optical sensor 230 can achieve an actual sensitivity that varieslongitudinally (e.g., from segment 235 to segment 237). Respectivesegments joined (e.g., in end-to-end abutment) as just described canprovide a sensor 230 having an actual sensitivity that varieslongitudinally, despite that the configuration of each segment can besubstantially identical to each other. Nonetheless, each segment canhave a unique configuration relative to one or more of the othersegments, providing a more pronounced difference in actual sensitivityfrom the other segments.

When such a sensor 230 extends among different environments 260 a, 260b, 260 c, 260 d (FIGS. 4, 6 and 7), variations in the apparentsensitivity of the sensor 230 can be substantially reduced compared to asensor having a longitudinally constant actual sensitivity extendingamong the environments.

For example, with a sensor as shown in FIG. 7, the actual sensitivity(and a corresponding reference-sensor pair) of the first segment 233 canbe selected to correspond with one or more characteristics of thecorresponding intended environment 260 a (e.g., underground). Thus, thesegment 233 can be configured to achieve a first apparent sensitivitywhen the sensor 230 is exposed to the environment 260 a. In a similarfashion, the actual sensitivity of the second segment 235 can beselected to correspond with one or more characteristics of thecorresponding intended environment 260 b (e.g., a wetland). Accordingly,the segment 235 can be configured (e.g., by selecting a reference-sensorpair configuration) to achieve a second apparent sensitivity when thesensor 230 is installed in the environment 260 b. The actual sensitivityof the third segment 237 and the fourth segment 239, respectively, canbe selected to correspond with one or more characteristics of thecorresponding intended environments 260 c (e.g., under water), 260 d(e.g., in the air) such that the segments 237, 239 achieve respectivethird and fourth apparent sensitivities when the sensor 230 isinstalled. The respective first, second, third and fourth apparentsensitivities can be more closely matched to each other thancorresponding portions of a sensor having a longitudinally constantactual sensitivity would exhibit when extending among the differentenvironments 260 a, 260 b, 260 c, 260 d.

Alternative Sensor Configurations

In some sensor embodiments, such as, for example, the sensor 230 shownin FIG. 4, the actual sensitivity of each respective segment differsfrom the actual sensitivity of each of the other segments; in otherinstances, the actual sensitivity of each of two or more respectivesegments is substantially the same. Also, although the sensor 230 isshown as having four segments 233, 235, 237, 239, alternative sensorembodiments can have more or fewer segments. The number of segments (andrespective actual sensitivities) can be selected to correspond to thenumber and types of environments the sensor is expected to be exposed toin use.

In addition, although segments 233, 235, 237, 239 are shown anddescribed as having six optical conduits circumferentially spaced fromeach other, other segment configurations are possible. For example, asegment can have more or fewer longitudinally extending optical conduitsthan shown in FIGS. 5A and 5B. Optical conduits can be spaced at otherthan 60-degrees from each other (even when six optical conduits arepresent in a given segment). A segment can have more or fewertight-buffered or loose-tube conduits.

Although “loose-tube fibers” and “tight-buffered fibers” are described,any suitable optical conduit can be used. Also, although opticalconduits exhibiting two classes of signal propagation or mechanicalcharacteristics (e.g., “loose-tube fibers” and “tight-buffered fibers”)are described, the disclosed principles apply to segments having a groupof conduits that exhibit more than two classes of signal propagationcharacteristics. Such characteristics include, by way of example,birefringence, length, phase, propagation time, polarization and coatingtype.

For a given segment, each pair of optical conduits selected as theoperative reference-sensor pair can provide the segment with a uniqueactual sensitivity. The range of achievable actual sensitivities cancorrespond, at least in part, to the physical and opticalcharacteristics and relative locations of each in the pair of conduits,as well as to the overall configuration (e.g., the number of opticalconduits in the bundle, the respective location of each conduit withinthe bundle, whether one or more interstitial spaces of the bundle isfilled, and if so, the material used to fill the spaces). For eachsegment in a multi-segment sensor (e.g., the sensor 230 shown in FIG.4), a respective pair of optical conduits can be selected as thereference-sensor pair based, at least in part, to correspond with aknown, or selected, environmental characteristics (e.g., materialproperties related to vibration-transmission through an environmentalmaterial, such as, for example, soil, water or air).

In some instances, a sensor can have a continually varying actualsensitivity along its length. In other instances, the sensor can have astepwise or discretely varying actual sensitivity along its length, aswith the sensor 230 shown in FIG. 4. In some instances, a sensor havingindividual segments with respective lengths less than or on the order ofa spatial resolution of the sensor can exhibit one or morecharacteristics of a sensor having a continuously varying sensitivity.

In some instances, a sensor can have only one optical conduit to make upthe reference-sensor pair (eg., a Sagnac interferometer or a modalmetricsensor). In other cases, more than two optical conduits may be used tocreate the sensor.

Other Embodiments

Using the principles disclosed herein, those of ordinary skill willappreciate a wide variety of possible embodiments of interferometersystems, particularly those configured to detect a disturbance with asensor extending among two or more environments. For example, althoughMichelson and Mach-Zehnder interferometers have been described above insome detail, sensors disclosed herein can be used with a variety ofother types of interferometers, such as, for example, overlapping firstand second Mach Zehnder interferometers, a Sagnac interferometer, amodalmetric sensor, an optical time domain reflectometer (OTDR), suchas, for example, a coherent-OTDR interferometer, a polarimeter, and manyother interferometer configurations.

This disclosure makes reference to the accompanying drawings which forma part hereof, wherein like numerals designate like parts throughout.The drawings illustrate specific embodiments, but other embodiments maybe formed and structural changes may be made without departing from theintended scope of this disclosure. Directions and references (e.g., up,down, top, bottom, left, right, rearward, forward, etc.) may be used tofacilitate discussion of the drawings but are not intended to belimiting. For example, certain terms may be used such as “up,” “down,”,“upper,” “lower,” “horizontal,” “vertical,” “left,” “right,” and thelike. These terms are used, where applicable, to provide some clarity ofdescription when dealing with relative relationships, particularly withrespect to the illustrated embodiments. Such terms are not, however,intended to imply absolute relationships, positions, and/ororientations. For example, with respect to an object, an “upper” surfacecan become a “lower” surface simply by turning the object over.Nevertheless, it is still the same surface and the object remains thesame. As used herein, “and/or” means “and” as well as “and” and “or.”

Accordingly, this detailed description shall not be construed in alimiting sense, and following a review of this disclosure, those ofordinary skill in the art will appreciate the wide variety ofinterferometer systems that can be devised and constructed using thevarious concepts described herein. Moreover, those of ordinary skill inthe art will appreciate that the exemplary embodiments disclosed hereincan be adapted to various configurations without departing from thedisclosed concepts. Thus, in view of the many possible embodiments towhich the disclosed principles can be applied, it should be recognizedthat the above-described embodiments are only examples and should not betaken as limiting in scope. And, although detailed claims have not beenpresented here since claims are not a necessary component for aprovisional patent application, I reserve the right to claim as myinvention all that comes within the scope and spirit of the subjectmatter disclosed herein, including but not limited to all that comeswithin the scope and spirit of the following paragraphs.

I reserve the right to claim, inter alia, the subject matter below:
 1. Ajoint in a fiber-optic cable, the joint comprising: a first opticalfiber having a first signal propagation characteristic; a second opticalfiber having a second signal propagation characteristic; wherein thefirst fiber and the second fiber are operatively coupled such that lightcan pass from one of the fibers to the other of the fibers, wherein anactual sensitivity changes from region adjacent the joint to another. 2.The joint of claim 1, further comprising a third optical fiberoperatively coupled to a fourth optical fiber such that light can passfrom one of the third or fourth optical fibers to the other of the thirdor fourth optical fibers.
 3. The joint of claim 2, further comprisingfirst and second segments, wherein the first optical fiber and the thirdoptical fiber comprise a reference-sensor pair in the first segment andwherein the second optical fiber and the fourth optical fiber comprise areference-sensor pair in the second segment, and wherein the firstsegment has a first actual sensitivity and the second segment has asecond actual sensitivity that differs from the first actualsensitivity.
 4. The joint of claim 3, wherein the third optical fiberand the fourth optical fiber have different signal propagationcharacteristics from each other.
 5. The joint of claim 3, wherein thefirst and third optical fibers have substantially identical signalpropagation characteristics.
 6. The joint of claim 3, wherein the secondand fourth optical fibers have substantially identical signalpropagation characteristics.
 7. The joint of claim 5, wherein each ofthe first and third optical fibers comprise respective loose-tube fibersor respective tight-buffered fibers.
 8. The joint of claim 1, whereinthe first optical fiber comprises loose-tube fibers and the secondoptical fiber comprises tight-buffered fibers.
 9. The joint of claim 1,further comprising an optical-fiber fusion splice or a mechanicalcoupling operatively coupling the first optical fiber and the secondoptical fiber.
 10. A fiber optic cable comprising: a first portioncomprising at least one fiber having a first signal propagationcharacteristic and at least one fiber having a second signal propagationcharacteristic; a second portion comprising at least one fiber havingthe first signal propagation characteristic and at least one fiberhaving the second signal propagation characteristic; and, an operativecoupling between the first portion's at least one fiber having the firstsignal propagation characteristic and the second portion's at least onefiber having the second signal propagation characteristic.
 11. The fiberoptic cable of claim 10, wherein the operative coupling comprises afirst operative coupling, wherein the fiber optic cable furthercomprises a second operative coupling between the first portion's atleast one fiber having a second signal propagation characteristic andthe second portion's at least one fiber having the first signalpropagation characteristic.
 12. The fiber optic cable of claim 10,wherein each fiber having the first signal propagation characteristiccomprises a loose-tube fiber.
 13. The fiber optic cable of claim 10,wherein each fiber having the second signal propagation characteristiccomprises a tight-buffered fiber.
 14. The fiber optic cable of claim 10,wherein a spacing between the at least one fiber having a first signalpropagation characteristic and the at least one fiber having a secondsignal propagation characteristic in the first portion differs from aspacing between the at least one fiber having a first signal propagationcharacteristic and the at least one fiber having a second signalpropagation characteristic in the second portion.
 15. The fiber opticcable of claim 10, wherein each fiber comprises a single-mode fiber. 16.The fiber optic cable of claim 10, wherein the at least one fiber havinga first signal propagation characteristic in the first portion comprisesa plurality of first loose-tube fibers.
 17. The fiber optic cable ofclaim 10, wherein the at least one fiber having a second signalpropagation characteristic in the first portion comprises a plurality offirst tight-buffer fibers.
 18. The fiber optic cable of claim 10,wherein each of the at least one fiber having a first signal propagationcharacteristic in the first portion and each of the at least one fiberhaving the first signal propagation characteristic in the second portioncomprises respective first and second pluralities of loose-tube fibers;wherein each of the at least one fiber having a second signalpropagation characteristic in the first portion and each of the at leastone fiber having the second signal propagation characteristic in thesecond portion comprises respective first and second pluralities oftight-buffer fibers.
 19. The fiber optic cable of claim 18, wherein theoperative coupling between the first portion's at least one fiber havingthe first signal propagation characteristic and the second portion's atleast one fiber having the second signal propagation characteristiccomprises a first operative coupling between one of the first loose-tubefibers and one of the second tight-buffer fibers.
 20. The fiber opticcable of claim 19, further comprising a second operative couplingbetween one of the first loose-tube fibers and one of the secondloose-tube fibers.
 21. The fiber optic cable of claim 19, furthercomprising a second operative coupling between one of the first tightbuffer fibers and one of the second loose-tube fibers.
 22. The fiberoptic cable of claim 19, further comprising a second operative couplingbetween one of the first tight buffer fibers and one of the secondtight-buffer fibers.
 23. The fiber optic cable of claim 18, wherein eachplurality of loose-tube fibers comprises two loose-tube fibers andwherein each plurality of tight-buffer fibers comprises fourtight-buffer fibers.
 24. The fiber optic cable of claim 23, wherein eachof the fibers defines a longitudinal axis and the cable furthercomprises an outer sheath defining a generally circular cross-sectionand a corresponding central longitudinal axis, and wherein each fiber'slongitudinal axis is positioned radially outward of the centrallongitudinal axis.
 25. The fiber optic cable of claim 24, wherein eachof the loose-tube fibers is positioned about 180-degrees from eachother.
 26. The fiber optic cable of claim 24, wherein each of thetight-buffer fibers is positioned about 180-degrees from one of theother tight-buffer fibers.
 27. An interferometer configured to detect adisturbance, the interferometer comprising: an active portion configuredto emit light into an optical sensor, to receive at least one opticalsignal from the optical sensor, or both; and an optical sensoroperatively coupled with the active portion, wherein the optical sensorcomprises a first segment having a first actual sensitivity and a secondsegment having a second actual sensitivity.
 28. The interferometer ofclaim 27, wherein the first segment is configured to provide a firstapparent sensitivity when exposed to a first environment and wherein thesecond segment is configured to provide a second apparent sensitivitywhen exposed to a second environment, wherein the first environmentdiffers from the second environment and wherein the first apparentsensitivity and the second apparent sensitivity are substantially thesame.
 29. The interferometer of claim 27, wherein the active portion andthe optical sensor are together configured to form a selected one of thegroup consisting of: (i) overlapping first and second Mach Zehnderinterferometers; (ii) a Sagnac interferometer; (iii) a modalmetricsensor; (iv) a coherent-OTDR; and (v) a polarimeter.
 30. Theinterferometer of claim 29, wherein the optical sensor is furtherconfigured to convey an optical such that a disturbance to a portion ofthe sensor tends to modify the optical signal.
 31. The interferometer ofclaim 30, wherein the active portion is further configured to monitorthe respective optical signals and to infer therefrom a location of adisturbance to the sensor.
 32. The interferometer of claim 29, whereinthe first segment is configured to provide a first apparent sensitivitywhen exposed to a first environment and wherein the second segment isconfigured to provide a second apparent sensitivity when exposed to asecond environment, wherein the first environment differs from thesecond environment and wherein the first apparent sensitivity and thesecond apparent sensitivity are substantially the same.
 33. Theinterferometer of claim 27, wherein the active portion and the opticalsensor are together configured to form a hybrid Michelson and MachZehnder interferometer.
 34. The interferometer of claim 33, wherein thefirst segment is configured provide a first apparent sensitivity whenexposed to a first environment and wherein the second segment isconfigured to provide a second apparent sensitivity when exposed to asecond environment, wherein the first environment differs from thesecond environment and wherein the first apparent sensitivity and thesecond apparent sensitivity are substantially the same.
 35. Theinterferometer of claim 27, further comprising one or more additionalsegments, each having a respective actual sensitivity different from thefirst actual sensitivity or the second actual sensitivity.
 36. Theinterferometer of claim 35, wherein each of the one or more additionalsegments is configured to provide a corresponding apparent sensitivitywhen exposed to a respective corresponding environment, and wherein eachcorresponding apparent sensitivity is substantially the same as thefirst apparent sensitivity and the second apparent sensitivity.